1. Technical Field
The invention relates to physical layer (PHY) digital signal processing for use in processors developed for wireless local area networks (LAN's), and more particularly to wireless LAN's based on orthogonal frequency division multiplexing (OFDM) in the license-free national information structure (U-NII) radio spectrum bands in the United States and generally conforming to IEEE Specification 802.11a.
2. Description of the Prior Art
Local area networks (LAN's) have traditionally been interconnected by twisted-wire pairs and shielded cables. However, there are several deficiencies of traditional LAN's. The main being restricted mobility. In contrast, a whole class of untethered computing has emerged which uses complex modulation and coding to achieve high-speed data rates. The IEEE 802.11a standard, herein “802.11a”, specifies, among other things, the physical layer (PHY) entity for an orthogonal frequency division multiplexing (OFDM) system with data payload communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s. The 802.11a standard specifies RF transmission in the 5.15-5.25, 5.25-5.35, and 5.725-5.825 GHZ unlicensed national information structure (U-NII) bands.
Typically, the IEEE communication standards specify the transmit bit-stream in addition to performance specifications, RF emissions requirements, etc. The wireless transmission medium inherently introduces some unique impairments (not present in traditional LAN's) to the transmitted signal which must be mitigated in the remote receiver station. These impairments include signal fading, multi-path reflections, base- and remote-unit oscillator mismatch introduced frequency offset, timing misalignment, and timing synchronization. In addition, there are RF hardware limitations such as receiver IQ imbalance and phase noise that must be mitigated as well. As such, the mitigation of such effects falls under the category of baseband digital signal processing. To assist the remote unit in mitigating these effects, a known training sequence is usually embedded into the transmit bit stream; this occurs at the expense of bandwidth. Of course, the same problems occur in the upstream direction (remote station transmitting to the base station), but it suffices to discuss the downstream digital signal processing.
In this disclosure, one such digital signal processing method, timing misalignment, is outlined. The term “timing misalignment” refers to the intra-baud timing offset between a received baud-rate sampled signal and what would be its on-baud sampled version. Without correction, the samples obtained are most likely to be off-baud and can cause serious degradation in demodulation. Note that the use of an equalizer supercedes the timing misalignment function when enabled. Therefore, the timing misalignment function can be used in certain situations only (when the transmission channel is simply described by a direct line-of-sight component and Gaussian noise for example).
It is assumed that some form of frequency correction has been applied to the signal prior to performing synchronization estimation. One embodiment of coarse frequency estimation is disclosed in “Coarse Frequency Offset Estimation—Patent Disclosure”, Alain Chiodini, John Reagan, nBand Communications, 2000. In addition, it is assumed that data synchronization so that a timing reference is known. One embodiment of timing synchronization is disclosed in “OFDM Data Demodulator Timing Synchronization—Patent Disclosure”, John Reagan, Alain Chiodini, nBand Communications, 2000.
Conventional methods for establishing timing misalignment estimation and correction are correlation with fractional baud misaligned reference signals. However, there are several disadvantages of this type of approach. These are: (a) several sequences need to be stored locally, i.e. the reference signal that is offset by k/N baud where k=−16:16, N=16, and (b) the prestored sequence does not account for signal distortions (A/D, quantization effects, phase distortion, IQ imbalance, . . . ) which results in non-optimal correlation values.
The transmission scheme in 802.11a is bursty. This means that the receivers must digitally process the training sequence to mitigate the undesired signal impairments each time a burst commences. This means that it is desirable for the processing blocks to be as robust and computationally efficient as possible. For example, to reduce computational complexity, the timing misalignment function can be used in lieu of an equalizer in situations when the transmission channel is simply described by a direct line-of-sight component and Gaussian noise.